The present invention relates to a voltage/current conversion circuit in which an output current is taken out which is proportional to an input voltage.
Shown in FIG. 1 is a prior art voltage/current conversion circuit in which an input voltage V.sub.i is applied to a non-inverting input terminal of an operational amplifier 11 and the output of the latter is applied to the base of a transistor Q.sub.1 constituting a current converting stage. The emitter of the transistor Q.sub.1 is connected both to a power supply V.sub.cc through a resistor R.sub.1 and to the inverting input terminal of the operational amplifier 11. The collector of the transistor Q.sub.1 is connected to the collector connected as a transistor Q.sub.2 of a diode. The transistor Q.sub.2, together with a transistor Q.sub.3 having a base commonly connected to the base of the transistor Q.sub.2, and emitter resistors R.sub.2 and R.sub.3 of the transistors Q.sub.2 and Q.sub.3, respectively, constitutes a current mirror circuit 12.
With such a circuit arrangement where a PNP transistor (Q.sub.1) is employed in the current converting stage, it is a usual practice to use a lateral PNP transistor due to the fact that a monolithic IC is relatively simple to fabricate in a IC manufacturing process. The lateral PNP is, however, disadvantageous in that transition frequency f.sub.T is low and is degraded in linearity of the current amplification factor h.sub.FE in a large current flow region, resulting that a preferrable characteristic has not been obtained.
Although the foregoing disadvantage can be resolved if a vertical PNP transistor is employed, it is disadvantageous in another aspects such that the IC manufacturing process becomes complicated involving a number of manufacturing steps, and therefore the cost for manufacturing the same becomes expensive.